76ページ 5行目 > Thus in ZF there is no set of all sets, nor any Russell set {x | not(x ∈ x)} (which > would be universal since ∀x(not x ∈ x) holds in ZF).
76ページ 下から4行目 > ZF and BG are both untyped formalisms, i.e. the levels which we have in mind in > their informal interpretations do not appear explicitly in the syntax. Thus we can > form expressions x ∈ y without restriction, in particular the expression x ∈ x.
